Position sensors monitor the position or motion of a first mechanical component relative to a second mechanical component, by producing an electrical signal that varies as the relative position of the two components. The relative speed of the two components can also be determined by determining the time derivative of the position. Electrical position sensors are an important part of innumerable products, and are useful for determining the status of various automotive actuations and processes that involve either linear or angular displacement. For example, the position of an adjustable automobile seat can be determined by position sensing devices mounted in the movable seat frame and the fixed seat guiding rails. The position and the angular velocity of the automotive engine crankshaft can also be determined by the appropriate placement of position sensing devices.
One prior art position sensor, a contacting position sensor, requires physical contact between a signal generator and a sensing element to produce an electrical signal representative of position. Contacting position sensors typically consist of a potentiometer electrically responsive to the signal generator and mechanically responsive to the component position, such that the output electrical signals vary as a function of the component's position. Motion-induced contact wear limits the durability of the contact-type position sensors.
Non-contact magnetic type position sensors determine the position of a moving object by measuring changes in a magnetic field. Ferromagnetic material disposed on a moving object passes through a constant magnetic field, modulating the field in accordance with the object's position. A field sensing device senses the field changes from which the object's position can be determined. One example of such a magnetic sensor includes a ferromagnetic target wheel attached to and rotated by a rotating object, the speed and/or position of which is to be determined. The target wheel comprises a plurality of ferromagnetic regions separated by regions of non-magnetic material. Typically the ferromagnetic regions are disposed adjacent the target wheel periphery. The magnetic field sensor comprises a stationary biasing magnet (conventionally a permanent magnet) positioned adjacent to the wheel periphery, and a magnetic field sensing device, such as a magnetoresistor or a Hall effect device, mounted proximate the stationary magnet. The constant magnetic field produced by the stationary biasing magnet is modulated by the ferromagnetic regions of the target wheel. As the wheel rotates, the ferromagnetic regions passes adjacent the stationary magnet, changing the reluctance of the magnetic circuit and in turn varying the magnetic flux density. These variations are sensed by the magnetoresistor and manifested as variations in the resistance thereof.
Electronic circuitry responsive to the magnetoresistor produces an analog signal that varies in response to the magnetic field flux density variations. Thus a voltage signal in the form of a DC-biased waveform is produced. The waveform characteristics correspond to the shape and spacing of the ferromagnetic regions. When the signal exceeds a predetermined threshold, a ferromagnetic region of the wheel has been detected adjacent the magnetoresistor. By appropriately spacing the ferromagnetic regions on the wheel, the angular position of the shaft can be determined. The angular velocity can also be determined as the rate of change of the position. It is known that the resistance of the magnetoresistor, and thus the position accuracy of such a device, is affected by the temperature, the air gap, magnet aging and the positional accuracy of the ferromagnetic regions relative to the rotating shaft.
In lieu of ferromagnetic teeth, a single continuous shaped ferromagnetic track, with a shape designed to produce continuous variations in the magnetic field as the target moves, can be used as a position sensor. A spiral shape is one example of a continuous target track. In one exemplary embodiment the target wheel comprises a substrate and a spiral ferromagnetic track formed thereon. For the exemplary spiral track geometry, the output signal is a linear function of the rotational angle. The angular velocity can also be determined as the rate of change of the position.
A Hall effect device can be used in lieu of a magnetoresistor to sense the changing magnetic field and provide an output signal in response thereto. As is known, a Hall effect device comprises a current-carrying conductor that when placed in a magnetic field, with the magnetic field flux lines perpendicular to the direction of current flow, generates a voltage across the device that is perpendicular to both the direction of current flow and the magnetic flux lines. Thus the Hall effect voltage, which is a function of the magnetic field flux density, serves as a position indicator for the ferromagnetic target.
Whether a magnetoresistor or a Hall effect device is utilized to sense the magnetic field and thus the object position, the position sensor must be accurate, in that it must produce an electrical signal based upon the measured position. An inaccurate position sensor hinders the proper position evaluation and control of the moving component. A position sensor must also be sufficiently precise in its measurement, although the degree of precision required depends upon the specific application. For some applications, only a rough indication of position is necessary. For instance, an indication of whether a valve is substantially opened or closed may be sufficient. In other applications a precise measurement of the valve position may be required. The position sensor must also be sufficiently durable for the environment in which it is placed. For example, a position sensor used on an automotive engine valve will experience almost constant movement while the automobile is in operation. The position sensor must therefore be constructed of mechanical and electrical components that allow it to remain sufficiently accurate and precise during its projected lifetime, despite considerable mechanical vibrations and thermal extremes and gradients.
The ferromagnetic targets discussed above are typically large and heavy structures, e.g., gears and slotted disks, manufactured by machining, stamping, blanking, powder metal technology, etc. These manufacturing methods are not only expensive, but are also not suitable for manufacturing targets with fine features and complex geometries that are required for high-accuracy small target sensors. Asymmetries in the placement of the teeth in a target wheel or changes in gap distance as the target wheel rotates cause inaccuracies in position determination.
Targets with precise features are particularly needed in state-of-the-art continuous linear and angular position sensors. Such continuous sensors determine position continuously over a range of values, such as angular rotation between 0° to 120°. By comparison, the toothed wheel sensors described above provide discrete position indications when a tooth passes adjacent the field sensing element. The continuous sensors employ a single shaped target where the shape is designed to produce continuous variations in the magnetic field as the target moves relative to the sensor. A spiral shape is one example of a continuous target. Although it is possible to manufacture precise continuous sensors using the prior art techniques of machining, stamping, etc. described above, precision equipment is required and thus the cost for such sensors is high.
One technique for forming precise ferromagnetic sensor targets is described and claimed in the co-pending, commonly-owned patent application entitled, Method for Forming Ferromagnetic Targets for Position Sensors, filed on Aug. 6, 2002, and assigned application Ser. No. 10/214,047. According to this method, photolithographic techniques allow for the formation of features as small as 0.1 mm by 0.1 mm, and up to about 1 mm thick for use with either discrete target or continuous target sensors. The magnetic field variations caused by targets with these dimensions can be sensed across air gaps in the range of about 0.25 to 0.5 mm, a range that is typical for high-accuracy position and speed sensors employed in most automotive systems.
In lieu of using ferromagnetic material, permanent magnet material can be used as the target track. According to prior art techniques, the formation of permanent magnet targets can be costly and is generally limited to simple geometries formed from discrete multipole magnets or from bulk permanent magnets. These techniques are not capable of satisfying the high accuracy requirements of today's state-of-the-art position and speed sensors. However, when using a permanent magnet material the bias magnet as described above and operative in conjunction with the magnetic field sensor is not required. Thus the cost of the bias magnet is avoided and the size of the sensing assembly is reduced.
According to the teachings of the co-pending commonly owned patent application entitled Method for Forming Permanent Magnet Targets for Position Sensors, filed on Feb. 24, 2004 and assigned application Ser. No. 10/372,750, a target track of magnetic material is formed using printed circuit board processing techniques. The magnetic field sensing can be performed by a Hall effect device, a magnetoresistor, etc., from which the target position information is derived.
To form a magnetic track according to the co-pending commonly-owned application, a photoresist material layer is disposed over the copper clad layer of conventional circuit board material. The photoresist layer is patterned and etched to form one or more trenches therein. The trenches are filled with a magnetic material through an electroforming process to form the target structures. The precision of the position detection is improved by the track's straight vertical sidewalls, resulting from use of the photolithographic process.